Chemically resistant polythioethers and formation thereof

ABSTRACT

A polythioether comprising:  
                 
 
     wherein R 1  is a C 1-10  alkyl, —(R 3 Q) p R 3 — or C 6 -C 20  aryl where Q is O or S, each R 3  is independently C 1-6  alkyl, and p is an integer between 0 and 6; R 2  is C 1-6  alkyloxy or C 5-12  cycloalkyloxy, R 4  is H, C 1-6  alkyl, C 1-6  alkyl alcohol and C 0-6  alkyl substituted with  
                 
 
     where X is a halogen, m is an integer between 1 and 4, and n is an integer selected to yield a molecular weight for said polythioether of between 1000 and 10,000 Daltons.

RELATED APPLICATIONS

[0001] This application is a divisional of U.S. patent application Ser.No. 09/802,427 filed Mar. 9, 2001, which claims priority of U.S.Provisional Patent Application Serial No. 60/188,106 filed Mar. 9, 2000.

FIELD OF THE INVENTION

[0002] The present invention relates to polythioethers and, moreparticularly, to polythioethers having a specified ratio of constituentatoms therein and a process for the formation thereof.

BACKGROUND OF THE INVENTION

[0003] Thiol-terminated sulfur-containing polymers have a long historyof use in aerospace sealants because of their fuel resistant nature uponcross-linking. Among the commercially available polymeric compoundshaving sufficient sulfur content to exhibit this desirable property arethe polysulfide polymers described, e.g., in U.S. Pat. No. 2,466,963 andsold under the trade name LP® polysulfide by Morton International ofChicago, Ill. and the alkyl side chain containing polythioether polymersdescribed, e.g., in U.S. Pat. No. 4,366,307 that are sold in completesealant formulations by PRC-DeSoto International, Inc. of Glendale,Calif. In addition to fuel resistance, polymers useful in this contextmust also have the desirable properties of low temperature flexibility,liquidity at room temperature, high temperature resistance, a reasonablecost of manufacture, and not be so malodorous as to prevent commercialacceptance of compositions that contain the subject polymers.

[0004] U.S. Pat. No. 4,366,307 teaches the use of hydroxyl-functionalthioethers having pendent alkylene groups to obtain polymers having goodflexibility and liquidity. However, the disclosed condensation reactionhas a maximum yield of about 75% of the desired condensation product.Furthermore, the acid-catalyzed reaction of beta-hydroxysulfidemonomers, such as thiodiglycol, yields significant quantities (typicallynot less than about 25%) of an aqueous solution of thermally stable andhighly malodorous cyclic byproducts, such as 1-thia-4-oxa-cyclohexane.As a result, the commercial viability of the disclosed polymers islimited. Further, pendent alkylene chains increase the carbon content ofthe polymer necessitating a high sulfur content to achieve sufficientchemical resistance properties.

[0005] U.S. Pat. No. 5,959,071 teaches the use of pendant alkylenechains and high sulfur content to achieve the chemical resistance androom temperature liquidity required for aerospace sealant formulations.

[0006] Certain prior art work has developed hydroxyl-terminatedpolythioethers by condensing thiodiglycol in the presence of certainetherifying catalysts as, for example, shown in U.S. Pat. Nos. 3,312,743and 3,335,189. Compounds produced by these patents give semi-crystallinewaxy solids, gums or low molecular weight liquids that have limitedcommercial utility.

[0007] Another desirable feature in polymers suitable for use inaerospace sealants is high temperature resistance. Inclusion ofcovalently-bonded sulfur atoms in organic polymers has been shown toenhance high temperature performance. However, in the polysulfidepolymers disclosed in U.S. Pat. No. 2,466,963, the multiple —S—S—linkages in the polymer backbones result in compromised thermalresistance. In the polymers disclosed in U.S. Pat. No. 4,366,307,enhanced thermal stability is achieved through replacement ofpolysulfide linkages with polythioether (—S—) linkages. In practice,however, the disclosed materials have compromised thermal resistance dueto traces of the residual acid condensation catalyst.

[0008] U.S. Pat. No. 5,912,319 teaches the use of combinations ofcertain polythiols with oxygenated dienes resulting in polythioetherpolymers that are liquids at room temperature and pressure and havedesirable physical properties. Further, these combinations aresubstantially free of residual catalysts and malodorous cyclicbyproducts. Unfortunately, the oxygenated dienes described are verydifficult to prepare and only a limited number of commercial compoundsare known to exist.

[0009] In addition to the foregoing deficiencies with the previouslyknown polythioethers, the prior art polythioethers are typically alsocrystallizing products which, even if liquid or semi-liquid at ambienttemperatures, when cooled sufficiently to solidify will not return totheir previous liquid state even when the temperature is raised toambient.

SUMMARY OF THE INVENTION

[0010] A polythioether comprising:

[0011] wherein R¹ is a C₁₋₁₀ alkyl, —(R³Q)_(p)R³— or C₆-C₂₀ aryl where Qis O or S, each R³ is independently C₁₋₆ alkyl, and p is an integerbetween 0 and 6; R² is C₁₋₆ alkyloxy or C₅₋₁₂ cycloalkyloxy, R⁴ is H,C₁₋₆ alkyl, C₁₋₆ alkyl alcohol and C₀₋₆ alkyl substituted with

[0012] where X is a halogen, m is an integer between 1 and 4, and n isan integer selected to yield a molecular weight for said polythioetherof between 1000 and 10,000 Daltons. A polyfunctionalizing agent isoptionally provided in order to increase the functionality of apolythioether from 2 to 4 with the most preferred range being 2.05 to3.00.

[0013] A process for forming such a polythioether includes the step ofreacting a polythiol with a polyhalide in the presence of an aqueousbase. The use of a polythioether is contemplated as an aerospacesealant.

DETAILED DESCRIPTION OF THE INVENTION

[0014] This invention consists of a polythioether polymer made by thereaction of a thiol (dithiol) with a polyhalide in an aqueous base. Thereaction may be illustrated as follows:

[0015] In the above illustration, “n” is a number such that a molecularweight of about 1000-10,000 Daltons is obtained resulting in materialhaving a viscosity of less than 1000 poise. R¹ is a C₁₋₁₀ alkyl,—(R³Q)_(p)R³— or C₆-C₂₀ aryl where Q is O or S, each R³ is independentlyC₁₋₆ alkyl, and p is an integer between 0 and 6. Substituents on R¹ arethose which do not interfere with the curing reaction of the polythiolwith either a polyepoxide or polydiene. Thus, R² is free of reactiveunsaturated carbon to carbon bonds, as well as highly water sensitivespecies. Preferably, R¹ is a linear alkyl. Preferred heteroatoms are Sand O. R² is a C₁-C₆ alkyloxy or C₅-C₁₂ cycloalkyloxy, most preferably—OCH₂CH₂— or —OCH₂OCH₂CH₂—. X is a halogen, F, Cl, Br or I and mostpreferably Cl.

[0016] Suitable thiols illustratively include ethanedithiol,vinylcyclohexyldithiol, dicyclopentadienedithiol, dipentene dimercaptan,and hexanedithiol. Preferred thiols contain heteroatoms. Examples ofwhich illustratively include dimercaptodiethyl sulfide (DMDS) with R¹ ofCH₂CH₂SCH₂CH₂, dimercaptodiethyl ether (DMDE) with R¹ of CH₂CH₂OCH₂CH₂and dimercaptodioxaoctane (DMDO) with R¹ of CH₂CH₂OCH₂CH₂OCH₂CH₂.

[0017] A polyhalide is chosen in conjugation with a polythiol to impartchemical resistance to the resulting polythioether, especially in thepresence of jet reference fuel 1 (JRF). Chemical resistance is manifestat a molecular level by maintaining the atomic percent ratio C:S:O inthe range 35-49:20-60:0-20. Thus, it will be apparent to those skilledin the art that pendant aliphatic moieties on the reactant polythiolsand polyhalides necessitates an increase in chain S content to maintainthe inventive polythioether atomic ratio and as such are generallydisfavored.

[0018] Suitable polyhalides illustratively include di- and tri-halidessuch as 1,2-dichloroethane and 1,2,3-trichloropropane. The blend ofhalide monomer in one mode of modifying polymer functionality.Oxygenated polyhalides are most preferred in order to maintain thecarbon atomic percent of the resulting polythioether below 50 percent.Examples of preferably polyhalides include Cl—CH₂CH₂OCH₂CH₂OCH₂CH₂—Cland Cl—CH₂CH₂OCH₂OCH₂CH₂—Cl.

[0019] The aqueous base used to prepare polymers of this inventionincludes solutions of both inorganic and organic bases. Suitableinorganic bases include, but are not limited to, sodium and potassiumhydroxide. Organic bases include any of a number of water soluble ordispersible tertiary amines such as pyridine.

[0020] In the representative reaction scheme illustrated above, thesubscript n is chosen such that the resulting polymer has a molecularweight, Tg, and room temperature viscosity within the requisite range,for example, of an aerospace sealant. Preferred molecular weightincludes 1000-6000 Daltons. In one embodiment, Tg should be as low aspossible but below −40° C. Finally, room temperature viscosity should bebelow 1000 poise.

[0021] While in the first preferred embodiment of the present inventionas summarized by Formula I, an excess of dithiol is present to assurethiol termination of the resulting polythioether polymer, it isappreciated that by varying the relative amounts of polythiol relativeto polyhalide, that polymers can be prepared that have not only terminalthiol groups, but a generic terminus R⁴ terminal where R⁴ is H, C₁₋₆alkyl, C₁₋₆ alkyl alcohol, and C₀₋₆ alkyl substituted with

[0022] Furthermore, the polythioether polymers thus formed need not befurther reacted prior to cure or, alternatively, are further reactedwith other compounds to form capped polythioether polymers. Capping ofpolythioethers of the present invention affords the opportunity toinclude additional terminal functionalities into the inventive polymers.Such capping moieties illustratively include hydroxyl, olefin, epoxy,cyano, isocyano, silyl, siloxy, secondary amine and alkyl groups.

[0023] In another embodiment, the inventive polythioether is greaterthan difunctional and is represented by the formula: B—(I—R⁴)_(z) whereB is a z-valent residue of a polyfunctionalizing agent, z is an integervalue from 3 to 6, I is the resulting polythioether of Formula I lessthe terminal hydrogen groups, and R⁴ is H, C₁₋₆ alkyl, C₁₋₆ alkylalcohol, and C₀₋₆ alkyl substituted with

[0024] Preferably, the average functionality, as defined as the numberof reactive groups per polythioether molecule, is above 2 and, morepreferably, ranges between about 2.05 and about 3.00.

[0025] In aerospace sealant applications, the polythioether polymerderived according to the invention may be combined with suitablelightweight fillers. Typically, a polythioether polymer is present atfrom about 40 to about 80 weight percent, 0.3 to 1.5 weight percentlightweight fillers or 10 to 50 weight percent of conventional inorganicfillers, 0.1 to 2 weight percent curing agent, and the remainder of thecomposition optionally including one or more additives of: pigments,cure accelerators, surfactants, adhesion promoters, thixotropic agentsand solvents. Suitable lightweight fillers for use in this invention maybe organic, inorganic, or a composite of both. They fall within twocategories—microspheres and amorphous fillers. The amounts of themicrospheres and amorphous lightweight fillers used in the compositionof the invention may be from about 0.3 to about 10 percent and fromabout 4 to 15 percent of the total weight of the composition,respectively. The specific gravity (s.g.) of the microspheres rangesfrom about 0.1 to 0.7 and are exemplified by polystyrene foam,microspheres of polyacrylates and polyolefins, and silica microsphereshaving particle sizes ranging from 5 to 100 microns and a specificgravity of 0.25 (ECCOSPHERES®, W. R. Grace & Co.). Other examplesinclude alumina/silica microspheres having particle sizes in the rangeof 5 to 300 microns and a specific gravity of 0.7 (FILLITE®,Pluess-Stauffer International), aluminum silicate microspheres having aspecific gravity of from about 0.45 to about 0.7 (Z-LIGHT®), and calciumcarbonate-coated polyvinylidene copolymer microspheres having a specificgravity of 0.13 (DUALITE 6001AE®, Pierce & Stevens Corp.).

[0026] Suitable amorphous lightweight fillers for use in this inventionhave a specific gravity ranging from about 1.0 to about 2.2 and areexemplified by calcium silicates, fumed silica, precipitated silica, andpolyethylene. Examples include calcium silicate having a specificgravity of from 2.1 to 2.2 and a particle size of from 3 to 4 microns(HUBERSORB HS-600®, J. M. Huber Corp.) and fumed silica having aspecific gravity of 1.7 to 1.8 with a particle size less than 1(CAB-O-SIL TS-720®, Cabot Corp.). Other examples include precipitatedsilica having a specific gravity of from 2 to 2.1 (HI-SIL TS-7000®, PPGIndustries), and polyethylene having a specific gravity of from 1 to 1.1and a particle size of from 10 to 20 microns (SHAMROCK S-395®, ShamrockTechnologies Inc.).

[0027] Table I illustrates the results of reacting various thiols andpolyhalides in aqueous base: TABLE I % Carbon Hydrogen Sulfur OxygenPhysical JRF Thiol Halide Yield* % % % % State Resistance DMDSCl—CH₂CH₂—Cl 97 40 7 53 0 Semi-solid Excellent DMDS Cl—CH₂CH₂OCH₂CH₂—Cl99 43 7 43 7 Liquid Excellent DMDS Cl—CH₂CH₂OCH₂CH₂OCH₂CH₂—Cl 98 45 7 3612 Liquid Good DMDO Cl—CH₂CH₂—Cl 99 46 8 31 15 Liquid Good DMDOCl—CH₂CH₂OCH₂CH₂—Cl 99 48 8 25 19 Liquid Acceptable DMDOCl—CH₂CH₂OCH₂CH₂OCH₂CH₂—Cl 98 49 8 21 22 Liquid Marginal

[0028] As Table I shows, according to the reaction process of theinvention, liquid polythioethers are produced without cyclizedbyproducts. In this manner, the tendency for production of highlymalodorous cyclic byproducts is greatly reduced. Further, monomer blendratios useful in the context of this invention are chosen such that thefollowing composition is preserved: carbon content, less than 49 percentby weight; sulfur content, 21-45 percent by weight; and oxygen content,less than 20 percent by weight. In this manner, the chemical resistanceproperties of the resulting polymer are suitable for applicationsincluding, but not limited to, aerospace sealants.

[0029] All patents and publications cited herein are hereby incorporatedby reference to the same extent as if each individual patent orpublication was explicitly and individually incorporated by reference.

1. A process for form-ing a polythioether comprising:

wherein R¹ is a C₁₋₁₀ alkyl, —(R³Q)_(p)R³— or C₆-C₂₀ aryl where Q is Oor S, each R³ is independently C₁₋₆ alkyl, and p is an integer between 0and 6; R² is C₁₋₆ alkyloxy or C₅₋₁₂ cycloalkyloxy, R⁴ is H, C₁₋₆ alkyl,C₁₋₆ alkyl alcohol and C₀₋₆ alkyl substituted with

where X is a halogen, m is an integer between 1 and 4, and n is aninteger selected to yield a molecular weight for said polythioether ofbetween 1000 and 10,000 Daltons comprising the step of: reacting apolythiol and a polyhalide in the presence of an aqueous base.
 2. Theprocess of claim 1 wherein said polythiol is a dithiol.
 3. The processof claim 2 wherein the dithiol is selected from the group consisting of:ethanedithiol, vinylcyclohexyldithiol, dicyclopentadienedithiol,dipentene dimercaptan, hexanedithiol, dimercaptodiethyl sulfide,dimercaptodiethyl ether, and dimercaptodioxaoctane.
 4. The process ofclaim 1 wherein said polyhalide is a dihalide.
 5. The process of claim 4wherein said dihalide is selected from the group consisting of:dichloroalkane and a polyglycol dihalide.
 6. The process of claim 1wherein said polyhalide is a polychloride.
 7. The process of claim 1wherein said polyhalide is a trihalide.
 8. The process of claim 1wherein n+1 equivalents of thiol of said polythiol are reacted with nequivalents of halide of said polyhalide.
 9. A curable compositioncomprising: 42 to 80 weight percent of a polythioether polymer accordingto claim 1, 0.3 to 15 weight percent of a lightweight filler and 0.1 to20 weight percent of a curing agent.
 10. The curable composition ofclaim 9 further comprising one or more additives selected from the groupconsisting of: pigments, cure accelerators, surfactants, adhesionpromoters, thixotropic agents and solvents.
 11. The curable compositionof claim 10 wherein said lightweight filler comprises microspheres. 12.The curable composition of claim 10 wherein said lightweight fillercomprises an amorphous material.